Turbine engines utilized in, for example, aircraft, have been started in any of a variety of ways. One typical starting scheme may utilize an air turbine connected in driving relation to the turbine engine. When it is desired to start the turbine engine, the air turbine is provided with air under pressure from an auxiliary power unit (APU) and driven until it in turn brings the turbine engine up to a self sustaining speed.
One difficulty with this approach is the fact that the air turbine is permanently connected to the turbine engine. Its presence thus adds weight to the aircraft reducing its useful load. Furthermore, in many instances, the presence of the air turbine may increase the frontal area of the turbine engine housing which thus increases aerodynamic drag, thus decreasing aircraft efficiency.
Electrical starters have also been utilized. Inasmuch as most aircraft of a size and capacity sufficient to justify the use of one or more turbine engines require a relatively high output electrical generating system to be driven by the turbine engine, there have been a number of proposals whereby the generator can be operated as a motor during an engine start mode and thus used to drive the turbine engine up to self sustaining speed. Power for the generator when utilized as a motor may be supplied by an APU or from the generating system of already running turbine engine in a multiple engine aircraft.
Electrical systems of this type have been fairly successful for their intended purpose. In many instances, however, the same utilize constant speed drives interconnecting the turbine engine and the generator and some mechanical provision must be made for operating a constant speed drive in reverse or bypassing the same during the engine start mode period. This generally requires the addition of mechanical components which, by reason of the nature of the forces involved, must be of fairly rugged construction and which in turn add weight to the aircraft. In some instances, the additional bulk of such components may even increase the frontal area of the engine housing leading to an increase in aerodynamic drag.
There are other types of generating systems employed in aircraft such as a so-called VSCF system. The name refers to a variable speed, constant frequency inverter system. It is desirable to provide a means for such a system whereby the generator in such a system can be alternatively used as a starter for starting an associated turbine engine. The present invention is directed to accomplishing that result in a system with minimum mass and a minimized efficiency penalty.